1. Field of the Invention
The present invention relates to a light pen detection system and more particularly to the one used for a display system of the raster scan type.
2. DESCRIPTION OF THE PRIOR ART
With recent prevelance of the process control by using electronic computers, monitoring items increase and controlled objects become large and complex. This background indispensably necessitates display systems as a means for man-to-machine communication. For the display systems, it generally is required to provide a write pen in order that an operator can easily communicate with the computer. The write or light pen is used for conveying the operator's intention to the computer through the picture of the display. Thus, it might be operable in an easy way and at a high precision. The causes of the erroneous operations of the light pen are:
A. Resolution of the light pen per se PA1 B. Various noises PA1 C. The light receiving face of the light pen incorrectly confronts the face of the CRT PA1 d. Parallax, i.e. a change in the position of the operator's eyes when he observes it PA1 E. When an ambiguous position (the boundary space between adjacent characters, for example) is indicated by the light pen PA1 F. Others
The erroneous operation due to causes (a) and (b) can be eliminated by precisely compensating for the character display position detected with some error and by using proper conventional noise prevention techniques. The causes (c) to (e) are all attributable to the operator's erroneous or improper operation of the light pen. The conventional light pen detection system is a so called one-touch and one-action system which is designed on the assumption that there is no occurrence of the erroneous operation by the operator as listed in causes (c) to (e). More precisely, in response to one switching operation of the light pen, the operation for detecting the character display position on the tube face is effected whether or not the character display position actually indicated by the light pen of the operator coincides with that desired by the operator.
However, it is very difficult to eliminate the parallax problem due to the change of the eye position with respect to the object for the reason that the face of the tube and the fluorescent screen are spaced 5 to 10 mm by the face plate and that the tube face is curved. Especially, in a high density display system in which huge numbers of characters are displayed on the tube screen, it would be almost impossible to obtain a correct indication by the light pen because of very small space between adjacent characters displayed. Thus, the existence of the adverse causes of (c) to (e) is inevitable at present-day technical level in this field. For this reason, the operator must perform the following troublesome operation. A desired character display position at first is provisionally detected and then the detected position is verified as whether or not it corresponds to the desired one. If this verification shows that it does not correspond to the desired one, such detecting operation will repeatedly continue until it will meet the desired one.